A bearing is a common part in machinery like wind turbines. Bearings are used to separate a rotating part of the machinery from a stationary part of the machinery. Bearings allow the rotation of the rotating part in respect to the stationary part while at the same time minimizing the friction between the two parts.
In wind turbines, bearings are used for several functions. In geared wind turbines, several bearings are used within the drive train for the hub, the main shaft, the gearbox and the generator. Bearings are also used in a wind turbine to allow the pitch movement of the rotor blades with respect to the hub.
Especially in direct drive wind turbines there is one main bearing carrying the rotor, including the hub, the rotor blades and a generators rotor of the wind turbine. This main bearing can be a bearing with a rolling element or a sliding bearing.
Such a main bearing is a major component of a direct drive wind turbine making up a considerable portion of total cost of the wind turbine. The exchange of such a main bearing bears extensive effort and cost, including a lifting operation to detach the hub from the bearing and gain access to the bearing.
To avoid the flow of electric currents through the bearing, the bearing is often electrically isolated from the rest of the machinery.
Electric currents can be creep currents, common mode currents, currents due to static charges or discharges in the case of lightning for example.
A conductive path bypassing the bearing is often provided to avoid the flow of electric currents, for instance from lightning, through the bearing.
Electric currents or discharges in the bearing i.e. electric sparks can lead to a degrading of the lubricant in the bearing, to the formation of hydrogen and carbonization of lubricant in the bearing. Also a static charge present in the bearing can lead to damage of the lubricant, especially due to electrolysis.
These can damage the lubrication of the bearing and can lead to a higher friction in the bearing. This can also lead to a damage of the surface of the bearing components by erosion and decomposition. This leads to mechanical problems in the bearing, like higher wear, thus the lifetime and the reliability of the bearing is reduced and the cost of service and maintenance are increased.
To avoid electric currents in the bearing the bearing is often electrically isolated from the rest of the machinery. This avoids that a current can flow through the bearing.
But electric charges can still build up between the machinery and the isolated part of the bearing. When the electric potential reaches a certain level the potential will discharge. This leads to short but strong current flowing through the bearing. This will lead to the same mechanical problems as described above.
WO2011069686 describes a lightning protection system for a wind turbine having an electrically grounded structure part, a main shaft and blades connected to the blade hub, comprises a down-conductor attachable inside a blade, a high voltage conductor for guiding lightning current, wherein a first end of the high voltage conductor is in electrical communication with the down-conductor. It further comprises a high voltage contact attachable inside the main shaft, wherein a second end of the high voltage conductor is in electrical communication with the high voltage contact and wherein a lightning current is guided from the down-conductor via the high voltage conductor to the high voltage contact.
This construction can reduce the risk of lightning strike currents leading through the bearing. If there is a failure in the connections of the high voltage conductor, the current of the lightning strike will lead through the bearing. Even if there is no failure, there is a certain possibility that the lightning will not choose the high voltage connection.
WO2011084723 describes an assembly, system and method for discharging a rotation shaft encompassed by a stationary housing. The system includes an annular charge-dissipating component and a biasing element oriented to apply an axial force to the charge-dissipating component, holding it in contact with the shaft and the housing while the shaft is rotating.
EP121546 describes an overvoltage protector in an apparatus having two moving parts. The parts are in mutual communication via a bearing, one part being stationary in relation to a substructure at an electrical earth potential. The moving part can be subjected to a voltage and the stationary part is electrically connected to the earthed substructure. For preventing a current passing through the bearing between the parts, an electrical insulator is provided in the current path which can occur through the bearing as a result of the different potentials of the parts. The moving part has a projection at a distance from a means connected to electrical earth such that a spark discharge gap is formed between mutually moving parts. The bearings are protected against current by the electrical insulator up to a given voltage threshold. In order that this voltage threshold will not be exceeded, the spark discharge gap comes into operation at a lower value that this voltage threshold and keeps the voltage difference between the parts at this lower value.
JP2005151749 describes a conducting member that conductively contacts a pair of an outer ring and a bearing housing and a pair of an inner ring, and is switchable in contacting and non-contacting. There is provided a switch drive means that switches the conducting member into a contact state and a noncontact state. There is also provided a switch control means that gives a switching command to the switch drive means.
All these solutions show the disadvantage of unreliability. The solutions mentioned above might get damaged by a first lightning strike and might lead to a current flowing through the bearing thereafter.